This invention relates to chemical-mechanical polishing (CMP) apparatus and, in particular, to a pneumatically actuated polishing head for such apparatus.
CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during the fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A polishing pad is typically constructed in two layers overlying a platen with the less resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is usually several times the diameter of a wafer and the wafer is kept off center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.
The platens used for a polishing pad and for a polishing head are carefully machined to produce optically flat, parallel surfaces. The resilient layers on a polishing pad are assumed to provide a uniform pressure on a wafer. It is believed that this assumption is in error and one aspect of the invention addresses the problem of distortion in the polishing pad.
Polishing heads must meet somewhat conflicting requirements for use in CMP apparatus. The wafer must be securely held but not damaged or contaminated. Polishing heads of the prior art typically use a wax-like material to attach the wafer temporarily to the carrier. The wax must be completely removable and must not affect the silicon to which it is attached. The polishing head cannot be so rigid that the wafer is chipped or damaged when the wafer engages the polishing pad or the head.
Gill, Jr. et al. U.S. Pat. No. 4,141,180 discloses a polishing head attached to a vertical shaft by a ball and socket joint to permit limited movement of the head to accommodate variations in the thickness of a wafer. The wafer carrier is covered with a felt-like material and the wafer is held against the nap surface by a vacuum coupled to the back of the wafer through a plurality of holes in the carrier. The polishing head includes a plastic ring encircling the wafer to locate the wafer radially with respect to the vertical shaft.
Shendon et al. U.S. Pat. No. 5,205,082 discloses a polishing head including a flexible diaphragm attached to a wafer carrier and to a retaining ring. Air pressure on one side of the diaphragm is above ambient pressure and air pressure on the wafer side of the diaphragm is at ambient pressure. The pressure on a wafer can be non-uniform due to forces from deflection of the diaphragm itself. Vertical motion of the carrier is limited by a flange engaging an adjustment bolt, which must be loosened to lower the carrier. A "C" washer is described for facilitating this adjustment.
The retaining ring surrounding a wafer in a polishing head of the prior art has an inside diameter slightly larger than the diameter of the wafer and there is always a slight gap between the wafer and the ring. Whether the ring presses against the resilient polishing pad or not, there is inevitably an annular region about the periphery of the wafer where the polishing is not uniform, known in the art as "edge exclusion." Edge exclusion in the prior art is typically 5-10 mm. wide and reduces the area of the wafer from which good die can be obtained.
It is known in the art that uniformity in wafer polishing is a function of pressure, velocity, and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The prior art attempts to solve the problem by contacting the polishing pad with the retaining ring, e.g. as disclosed in the Shendon et al. patent, but the problem remains.
Another aspect of the problem of uniformity is the distribution of the slurry. As a wafer is polished, chemical by-products locally change the composition, pH, particle size, and uniformity of the slurry. In the prior art, this problem was addressed by thoroughly mixing the slurry and by controlling the rate of flow to the polishing pad; specifically, by providing a sufficient flow to prevent large, local changes in composition, particle size, or pH. Even so, edge exclusion remains a problem.
In view of the foregoing, it is therefore an object of the invention to provide an improved polishing head for CMP apparatus.
Another object of the invention is to provide a pneumatic polishing head in which concentric pistons locate a wafer vertically and radially.
A further object of the invention is to provide a polishing head in which the ring surrounding a wafer is pressed against the polishing pad with a force independent of the force applied to the wafer.
Another object of the invention is to provide a more uniform pressure across a wafer, particularly at the edge of the wafer.
A further object of the invention is to improve circulation of slurry to minimize non-uniformities in concentration across the surface of a wafer.
Another object of the invention is to provide a wafer polisher that produces smaller edge exclusion than polishers of the prior art.
A further object of the invention is to provide a polishing head that distorts the polishing pad less than polishing heads of the prior art.
Another object of the invention is to provide a retaining ring that assists in circulating slurry across a wafer to minimize local variations in composition or pH.